1. Field of the Invention
The present invention relates to a method for producing an aromatic polycarbonate. More particularly, the present invention is concerned with a method for producing an aromatic polycarbonate, which comprises treating a molten aromatic polycarbonate prepolymer (obtained by reacting an aromatic dihydroxy compound with a diaryl carbonate) with an inert gas under a predetermined pressure to thereby cause the molten prepolymer to absorb the inert gas, and subjecting the resultant molten prepolymer having the inert gas absorbed therein to polymerization, under a pressure lower than the above-mentioned predetermined pressure employed for the inert gas absorption, to thereby polymerize the prepolymer to a predetermined degree of polymerization. According to the method of the present invention, it is possible to produce a high quality aromatic polycarbonate, which not only is colorless, but also has excellent mechanical properties, at high polymerization rate even without using a large amount of an inert gas. Further, according to the method of the present invention, even if the production of an aromatic polycarbonate is conducted in a continuous manner, the molecular weight of the produced aromatic polycarbonate can be maintained at a desired level, so that the above-mentioned high quality polycarbonate can be stably produced for a prolonged period of time. Therefore, the method of the present invention is commercially very advantageous.
2. Prior Art
In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound and phosgene has been commercially practiced, wherein 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as "bisphenol A") can be mentioned as a representative example of the aromatic dihydroxy compound.
However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous, that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as hydrogen chloride and sodium chloride, which are by-produced, and methylene chloride which is used as a solvent in a large quantity, and that difficulties are encountered in separating and removing impurities (such as sodium chloride) and residual methylene chloride, which adversely affect properties of a produced polymer.
As a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, a melt transesterification process has conventionally been known, in which an aromatic polycarbonate is produced by performing an ester exchange reaction between an aromatic dihydroxy compound (such as bisphenol A) and a diaryl carbonate (such as diphenyl carbonate) in the molten state, while removing by-produced phenol from the equilibrium polycondensation reaction system. Contrary to the interfacial polycondensation process, the melt transesterification process has an advantage in that a solvent need not be used. However, the melt transesterification process has the following serious problem: namely: since the viscosity of a polymer being formed increases during the progress of the polymerization reaction, it becomes difficult to remove efficiently by-produced phenol from the polymerization reaction system, thus making it difficult to achieve a high degree of polymerization with respect to a polycarbonate produced.
Various polymerizers have been known for use in producing aromatic polycarbonates by the melt transesterification process. A vertical agitation type polymerizer vessel equipped with an agitator is widely used. The vertical agitation type polymerizer vessel equipped with an agitator is advantageous in that it exhibits high volumetric efficiency and has a simple construction, so that polymerization on a small scale can be efficiently carried out. However, the vertical agitation type polymerizer vessel has a problem in that, as mentioned above, the by-produced phenol becomes difficult to remove efficiently from the polymerization reaction system in the production of aromatic polycarbonates on a commercial scale, so that the polymerization rate becomes extremely low.
Specifically, a large-scale vertical agitation type polymerizer vessel generally has a greater ratio of the liquid volume to the vaporization area than a small-scale one. In other words, the depth of a reaction mixture in the agitation type polymerizer vessel is large and, hence, the pressure in the lower part of the agitation type polymerizer vessel is high. In such a case, even if the degree of vacuum of the polymerization reaction zone is increased in order to achieve a high degree of polymerization in the lower part of the agitation type polymerizer vessel, the polymerization proceeds under high pressure due to the weight of the reaction mixture, so that phenol and the like cannot be efficiently removed.
To solve the above-mentioned problem, various attempts have been made to remove phenol and the like from a high viscosity polymer being formed. For example, Examined Japanese Patent Application Publication No. 50-19600 (corresponding to GB-1007302) discloses the use of a screw type polymerizer device having a vent. Examined Japanese Patent Application Publication No. 52-36159 discloses the use of an intermeshing twin-screw extruder. Examined Japanese Patent Application Publication No. 53-5718 (corresponding to U.S. Pat. No. 3,888,826) describes the use of a wiped film evaporation type reactor, such as a screw evaporator or a centrifugal film evaporator. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923 discloses a method in which a combination of a wiped film evaporation type apparatus and a horizontal agitation type polymerizer vessel is used.
The present inventors have developed a free-fall polymerization process in which a prepolymer is allowed to pass downwardly through a perforated plate and fall freely, so that the polymerization of the prepolymer is effected during the free fall thereof, and a guide-wetting fall polymerization process in which a prepolymer is allowed to fall along and in contact with the surface of a guide, so that the polymerization of the prepolymer is effected during the fall thereof (see, for example, U.S. Pat. No. 5,589,564).
It is widely known to effect a polymerization in an atmosphere of an inert gas in the production of aromatic polycarbonates by the melt transesterification process using the above-mentioned polymerizer devices, such as the vertical agitation type polymerizer vessel, the intermeshing twin-screw extruder and the wiped film evaporation type reactor. For example, U.S. Pat. Nos. 2,964,297 and 3,153,008 describe a method in which the production of an aromatic polycarbonate by the melt transesterification process is conducted under reduced pressure in an atmosphere of an inert gas so as to prevent occurrence of an oxidative secondary reaction (i.e., oxidation of the polymer), wherein the inert gas is introduced into a polymerizer device in a small amount, relative to the amount of the aromatic polycarbonate to be produced.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-206997 (corresponding to U.S. Pat. No. 5,384,389) describes a method for producing an aromatic polycarbonate in which an aromatic monohydroxy compound, such as phenol, by-produced in the equilibrium polycondensation reaction (for producing an aromatic polycarbonate) is removed from the polymerization reaction system by using a large amount of an inert gas. Specifically, in this method, an aromatic polycarbonate is produced by performing the equilibrium polycondensation reaction while continuously introducing an inert gas, together with a molten oligocarbonate, into a heated polymerizer device under atmospheric pressure or under superatmospheric pressure in an amount of 1 m.sup.3 or more per kg of the oligocarbonate, and removing by distillation phenol or the like (which is by-produced in the equilibrium polycondensation reaction) in such a form as entrained by the inert gas. However, such a method (in which the production of an aromatic polycarbonate is conducted while removing the by-produced aromatic monohydroxy compound, such as phenol, from the polymerizer device by using a large amount of the inert gas) has a problem in that, for recycling the inert gas used in the polymerization reaction, it is necessary to separate the aromatic monohydroxy compound (such as phenol) from the inert gas by using a large separation apparatus.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-248067 describes a method for producing an aromatic polycarbonate, which comprises producing a low molecular weight polycarbonate having a viscosity average molecular weight of from 1,000 to 25,000 in a first reaction zone, and polymerizing the produced low molecular weight polycarbonate in a second reaction zone to thereby obtain a final polycarbonate having a viscosity average molecular weight of from 10,000 to 50,000, wherein an inert gas is introduced into the first reaction zone and the second reaction zone in amounts of 0.01 to 20 and 0.002 to 10, respectively, each in terms of the weight ratio of the inert gas to the aromatic dihydroxy compound. Also in this method, the inert gas is used for removing by distillation the aromatic monohydroxy compound (such as phenol), which is by-produced in the equilibrium polycondensation reaction, from the reaction system in such a form as entrained by the inert gas, so that the inert gas is used in a large amount. Therefore, this method has the same problem as mentioned above in connection with Unexamined Japanese Patent Application Laid-Open Specification No. 6-206997.
In the above-mentioned U.S. Pat. No. 5,589,564 disclosing the method which has been developed by the present inventors and in which the polymerization of a prepolymer is conducted by allowing the prepolymer to pass downwardly through a perforated plate and fall along and in contact with the surface of a guide, it is described that a colorless, high quality polycarbonate can be produced at high polymerization rate. In the Working Examples of this patent document, a small amount of an inert gas is introduced into the polymerizer device. Further, in Unexamined Japanese Patent Application Laid-Open Specification No. 8-325373, the present inventors proposed a method in which the polymerization of a prepolymer is conducted by allowing the prepolymer to fall along and in contact with the surface of a guide, wherein the ratio of the partial pressure of the aromatic monohydroxy compound contained in the inert gas to the partial pressure of the inert gas, is controlled within such a specific range that the recovery of the inert gas can be conducted even without using a large apparatus.
In conventional methods in which an attempt is made to produce an aromatic polycarbonate by efficiently withdrawing phenol and the like from the polymerization reaction system by using an inert gas, as in the cases of the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification Nos. 6-206997 and 6-248067, the inert gas is continuously introduced into a polymerizer so as to reduce the partial pressure of phenol and the like in the polymerizer device, to thereby advance the polymerization reaction. In the above-mentioned methods, however, for effectively increasing the polymerization rate, it is necessary to use a large amount of the inert gas. Further, with respect to all of the above-mentioned methods using a large amount of the inner gas (in which the inert gas is introduced into only a polymerizer device to thereby remove by distillation the by-produced aromatic monohydroxy compound in such a form as entrained by the inert gas), such methods are not satisfactorily effective for constantly producing an aromatic polycarbonate having not only a desired molecular weight, but also excellent mechanical properties.